The present invention relates to amplifier circuitry, and more particularly, to amplifier circuitry for driving large capacitive loads.
Buffers and amplifiers are widely used building blocks in many circuit applications. A number of classes of amplifiers have been established to meet the needs of various applications. Class A amplifiers, for example, are linear amplifiers in which the output current flows over the whole range of the input voltage. Class A amplifiers are said to have low distortion, but also low efficiency. Class AB amplifiers are amplifiers that deliver to and pull from a load a current that is larger than the DC quiescent current flowing in the class AB circuit. At low input signal levels, class AB amplifiers tend to operate as class A amplifiers. In addition, class AB amplifiers should have low, controlled DC output quiescent current. With proper design, the output current of a class AB amplifier should increase when a large differential voltage is applied. For a more detailed discussion of such classes of amplifiers, see, for example, P. R. Gray and R. G. Meyer, xe2x80x9cAnalysis and Design of Analog Integrated Circuits,xe2x80x9d 755-756 (2d ed. 1984).
Single-stage class AB amplifiers have been widely used to obtain good settling characteristics for a buffer that drives large capacitive loads. Class AB amplifiers and buffers often form part of a larger on-chip system that interfaces with external devices and are typically fabricated using CMOS transistor technology. As the CMOS processes are scaled to increase the circuit density for a given area of silicon, there is a corresponding increase in the complexity of the circuit design. In order to minimize the power dissipation, such CMOS circuits must operate at increasingly lower voltages, such as three volts (3 V), 1.8 V or even lower. Thus, there is a need to operate the class AB amplifiers or buffers contained on a CMOS chip using lower supply voltages, while driving large external capacitive loads. A video cable, for example, can provide a capacitive load on the order of 100 picofarads (pF).
Generally, a class AB buffer (or amplifier) is disclosed for driving a large capacitive load. The disclosed CMOS class AB buffer can drive capacitive loads, for example, in excess of 100 pF, while operating from a voltage supply as low as 1.5 volts. In addition, the disclosed CMOS class AB amplifier or buffer consumes very little quiescent current, while exhibiting a non-slewing transient response.
The disclosed class AB buffer includes a pair of driving transistors that are cross-coupled through an amplifier and level shifting circuitry, such as transistor circuitry, and a pair of current source transistors each having a gate terminal connected to an output of the corresponding amplifier and a gate terminal of an output transistor, and a drain terminal connected to a source terminal of the driving transistors. In this manner, maximum current can be obtained when the current source transistors are in the linear region (and are driving the output transistors that are typically in the saturation region). According to one aspect of the invention, the driving transistors are prevented from entering a linear region by connecting a drain terminal of each of the driving transistors to a positive power supply voltage. According to another aspect of the invention, only the threshold voltage of one transistor must be overcome before the transistors conduct current, since the gate-sources of the driving and current source transistors are not in series.
The performance of the disclosed class AB buffer may be further improved by using cascode p-channel transistors in an output stage to increase the gain. In one implementation, the cascode p-channel transistors are biased dynamically to maximize the output swing of the class AB buffer. In addition, a cascode current source can be used in an input stage between each of the driving transistors and the current source transistors to bias the driving transistors at the verge of saturation. The stability of the circuit is ensured by selecting the capacitance of the load to ensure that a first non-dominant pole of the class AB buffer is greater than the unity gain bandwidth of the class AB buffer over substantially all operating conditions.